Sequels are almost never as good as the original movie, so it's a downright miracle when a franchise gets better as it goes along. It's hard to think of many that achieved that feat, but here's one you may not have considered: The Paranormal Activity series.

Paranormal Activity 4 comes out in a week; I haven't yet seen it, but I hope it can somehow keep the winning streak alive. By this point in a franchise—horror or otherwise—filmmakers are usually spinning their wheels, desperately introducing new characters, and completely losing the essence of what made the original film special in the first place. But each film in the Paranormal Activity franchise has successfully riffed on the original, expanding the universe slowly and smartly.

When the first Paranormal Activity came out in September 2009, its buzz built gradually. Soon the movie became a word-of-mouth sensation. Here was a horror movie with no stars made to look like a collection of recovered footage shot by a Southern California couple (Katie Featherston and Micah Sloat) who became increasingly concerned that their place was haunted. Featuring naturalistic performances, stripped-down effects and little gore, Paranormal Activity was the exact opposite of the torture-porn films made popular by the brutal, melodramatic Saw movies. And in the process, Paranormal Activity became one of the most profitable movies of all time.

Naturally, Paramount decided to make a sequel, but the man behind the original Paranormal Activity, Oren Peli, took a backseat for the followup film. Instead, they went with new writers and a new director, Tod Williams, who had never made a horror movie before. (Previously, he had directed the Jeff Bridges drama The Door in the Floor.)

You'd assume that because Paranormal Activity 2 basically copied the strategy of the first film, it wouldn't be nearly as interesting. Instead, the opposite was true. While some of the freshness was gone, 2 fell like a revised, improved version of 1—a final draft compared to 1's rough sketch. As clever as Peli's original idea was, the first film suffered once you got past the found-footage conceit: The acting wasn't so hot, and the storytelling was spotty. (In particular, Micah was such an obnoxious character—and Katie was so clearly in the right about being freaked out about the house's nocturnal terrors—that it was hard to understand why she was with such a dolt.) 2 had better performances and actually went to the trouble of developing the characters a little. (It's a small thing, but 2 creates some of its tension by the fact that Sprague Grayden's Kristi—Katie's sister—feels slightly ganged-up-on by her much-older husband and his daughter Ali. It helps add to Kristi's paranoia when she's doing battle with invisible demons.) And, probably most impressive of all, 2 figured out how to tie itself into 1's storyline in a way that didn't feel forced, actually adding to our understanding of what happened in the first film.

Still, there was no reason to expect much from Paranormal Activity 3, especially when they decided to go back in time to when Kristi and Katie were kids, which smacked of moneymaking desperation. Of course! A prequel! But once again, the producers switched things up with new directors, Henry Joost and Ariel Schulman, the team behind the controversial documentary Catfish. Regardless of Paramount's intent, it was a cheeky move: They chose the guys whose nonfiction film some people thought was fake to direct their horror movie that pretended to be real. 3 has a lot working against it—you already know what happens to both sisters, you're very familiar with the movie's format—yet it produced the most consistent scares of the series. Part of that has to do with its ending, the best in the franchise—while I'm not sure any character in that situation needs to be running around with a camera, god, does it work well—and part of it has to do with the simply inspired choice to have the characters set up a camera on top of an oscillating fan, which creates this brilliant now-you-see-it/now-you-don't sense of terror as weird happenings occur in the frame and then move out of frame:

Now that we're on Paranormal Activity 4, this series isn't so much about its found-footage conceit anymore. Instead, the sequels have taken what's great about that original concept and built on it, streamlining the terror of knowing that somewhere in the frame something terrible will happen, but not knowing where for sure. By this point, it's become something of a game between the filmmakers and the audience—they know that we know how these movies work—so the fun comes in seeing how they're going to get us this time.

Just to be clear, I know that the Paranormal Activity movies have clear flaws, not the least of which is, uh, who is editing this found footage? But the sequels' ability to adhere to the original's less-is-more technique while still scaring us is a high-wire act that gets more amazing as it goes on. 4 will be the first time the series has repeated a previous installment's writers and directors. (Joost and Schulman are back—as is Christopher Landon, who co-wrote 2 and wrote 3.) I'm actually forcing myself not to learn anything about 4's plot, but I do know that Adult Katie is once again involved. And once again I'm suspicious they won't be able to pull it off. But that's the way I've felt going in to every one of these sequels—and each time they've proved me wrong. With this franchise, I don't ever want to be right.

Grierson & Leitch is a regular column about the movies. Follow us on Twitter, @griersonleitch.